Electricity and magnetism Flashcards
(124 cards)
1
Q
Describe the forces between magnets
A
Magnets attract or repel other magnets
2
Q
Describe the forces between magnets and magnetic materials
A
Magnets will always attract other magnetic materials
3
Q
What causes magnetic forces?
A
Interactions between magnetic fields
4
Q
What are 3 magnetic materials (ferrous)?
A
Iron, cobalt, nickel
5
Q
What are 3 non-magnetic materials (non-ferrous)?
A
Plastic, aluminium, copper
6
Q
What are 3 methods of magnetisation?
A
- Stroking with a magnet
- Use of direct current (d.c.) in a coil
- Hammering in a magnetic field
7
Q
What are 3 methods of demagnetisation?
A
- Hammering
- Heating
- Use of alternating current (a.c.) in a coil and slowly remove the material
8
Q
What are magnetic field lines?
A
- A way of picturing the space around a magnet
2. They show its strength and direction
9
Q
Describe an experiment to identify the pattern of magnetic field lines, including the direction
A
- Place the magnetic on plain paper
- Draw around the magnet using a pencil
- Make a dot one end
- Place a compass needle in line with the dot
- Make a dot where the compass needle points to
- Point the compass needle to the next point
- Make 4 more dots and connect them
- Repeat with the compass needle on other points along the magnet
10
Q
What are 2 rules to follow when drawing magnetic field lines?
A
- They always go from north to south
2. They never touch or cross other field lines
11
Q
What are 2 magnetic properties of soft iron?
A
- Easy to magnetise
2. Easy to demagnetise
12
Q
What are 2 magnetic properties of steel?
A
- Hard to magnetise
2. Hard to demagnetise
13
Q
What are permanent magnets made out from?
A
Hard magnetic materials
14
Q
What are electromagnets made out from?
A
Soft magnetic materials
15
Q
What is meant by the term hard magnetic material?
A
Magnetic materials that can be permanently magnetised
16
Q
What is meant by the term soft magnetic material?
A
Magnetic materials that can only be temporarily magnetised
17
Q
What is the design and use of a permanent magnet?
A
- Hard magnetic material
2. Fridge doors
18
Q
What is the design and use of an electromagnet?
A
- Solenoid
2. Scrap metal moving
19
Q
What are the 2 types of charges?
A
Positive and negative
20
Q
What happens between unlike charges?
A
Attraction
21
Q
What happens between like charges?
A
Repulsion
22
Q
What is the SI unit for charge?
A
Coulombs (C)
23
Q
Describe simple experiments to show the production and detection of electrostatic charges
A
- Suspend one of the materials using a cradle and a length of spring
- Rub on end of the material using a cloth
- Rub the second piece of insulating material using a cloth
- Hold the charged end of the second piece close to the charged end of the suspended piece
- If the first piece is repelled, the materials have the same charge
24
Q
What does charging a body involve?
A
Addition or removal of electrons
25
What is the direction of an electric field at a point?
The direction of the force on a positive charge at that point
26
Describe an electric field?
A region in which an electric charge experiences a force
27
Describe charging by induction
1. Charged object is held close to the conductor
| 2. Electrons in the conductor are able to move towards or away from the object
28
What is an electrical conductor?
A material that allows charge to flow through it
29
What is an electrical insulator?
A material that does not allow charge to flow through it
30
What are 2 examples of electrical conductors?
Metals and graphite
31
What are 2 examples of electrical insulators?
Non-metals and plastics
32
What is an electron?
A negatively charged particle
33
What is current?
Rate of flow of charge
34
What is used to measure current?
Ammeter
35
What is the equation for current?
I = Q ÷ t
36
What causes current in metals?
Flow of electrons
37
What is the SI unit of current?
Amperes (A)
38
How is an ammeter connected to a circuit?
Always in series
39
What is conventional current?
Positive to negative
40
What is the electromotive force (e.m.f.) of an electrical source of energy measured in?
Volts (V)
41
What is the definition of e.m.f.
Energy supplied by a source in driving 1C of charge round a complete circuit
42
What is potential difference (p.d.) measured in?
Volts (V)
43
What is 1 V equivalent to?
1 J/C
44
What is used to measure potential difference?
Voltmeter
45
What is potential difference?
Amount of energy transferred by each unit of charge passing between those two points
46
How is a voltmeter connected to a circuit?
In parallel with the part of the circuit you want to measure the potential difference of
47
What is resistance?
Opposition to current
48
What is the formula for resistance?
R = V ÷ I
49
Describe an experiment to determine resistance using a voltmeter and an ammeter
1. Set the power supply to a low voltage (1 or 2 V)
2. Use a voltmeter and ammeter to obtain voltage and current readings
3. Substitute readings into the equation R = V ÷ I
50
What is the relationship between length and resistance?
Length is proportional to resistance
51
What is the relationship between cross-sectional area of a wire and resistance?
Cross-sectional area of a wire is inversely proportional to resistance
52
What is the flow of energy in electric circuits?
Battery/power source → circuit components → surroundings
53
What is the formula for power?
P = VI
54
What is the formula for energy transferred?
E = Pt / E = VIt
55
Describe the current in a series circuit?
The same at every point
56
What is the sum of combined resistances in a series circuit equal to?
The sum on individual resistances
57
What is the sum of the p.d.s across the components in a series circuit equal to?
Total p.d. across the supply
58
Describe current in a parallel circuit?
Current from the source is larger than the current in each branch
59
Describe combined resistance of two resistors in parallel?
Less than that of either resistor by itself
60
What is current from the source in a parallel circuit equal to?
Currents in separate branches
61
What are the advantages of connecting lamps in parallel in a lighting circuit?
1. Lamps can be individually controlled, using their own switches
2. If one lamp stops to function the other will continue to work
3. Each lamp gets maximum p.d.
62
What is a variable potential divider also known as?
Potentiometer
63
What is the relationship between p.d. and resistance across 2 resistors?
The resistor with a largest resistance will have greater p.d. than the other
64
What does a potentiometer consist of?
A coil of wire with a sliding contact along it
65
What does moving the sliding contact do in a potentiometer?
1. Increases or decreases the length of wire
| 2. Increases or decreases resistance (and hence p.d.)
66
What is a thermistor?
A resistor that changes its resistance when it is heated
67
What is the relationship between temperature and resistance in thermistors?
Temperature is inversely proportional to resistance
68
What is a light-dependent resistor?
A resistor that changes its resistance when light shines upon it
69
What is the relationship between light and resistance in an LDR?
Light is inversely proportional to resistance
70
What is the formula to find V.out from a potentiometer?
V.out = V.in × (R2 ÷ R.Total)
71
What is the function of a transducer?
Takes in physical energy and converts it into an electrical signal that can be read
72
What does a relay consist of?
A coil and magnetic switch
73
Describe the action of a relay
1. When there is a current in the coil it creates a magnetic effect
2. This attracts the switch, closing it
3. This can be used to control a switch in a separate circuit
74
What is a diode?
A component which only allows charge to flow in one direction through it
75
In what direction must the diode point to to have a current?
Positive to negative
76
What does a diode do to an a.c. power supply?
Rectification
77
What is rectification?
Conversion of a.c. to d.c.
78
How is an LDR used, along with a relay, to control a bulb?
| *Light sensitive switch
1. When it gets darker, resistance increases
2. This causes an increase in p.d.
3. Increase in p.d. causes an increase in current in the coil
4. A large enough current in the coil will attract the switch, closing it
5. This allows a current to pass through
79
Exam tip ; )
Temperature-controlled (temperature-operated alarm) circuits work the same way as LDR-controlled circuits
80
What are the 2 types of signal?
Analogue and digital
81
What is an analogue signal?
One that is continuously varying and can take any value
82
What is a digital signal?
One that can only take one of two (discrete) states
83
What is a logic gate?
An electronic component that gives a particular output, depending on the input(s) to it
84
What is the hazard of damaged insulation?
Electric shock
85
What is the hazard of overheating of cables?
Fire
86
What is the hazard of damp conditions?
Electrocution
87
What does a fuse do?
Protects a circuit
88
Explain the use of fuses
1. If the current in the wire becomes too large
2. The wire heats up and melts
3. The wire breaks, stopping the current and breaking the circuit
89
Explain choosing appropriate fuse ratings
The fuse should always have a higher current rating than that needed by the appliance - without being too high
90
Explain the use of circuit breakers
An automatic switch which
1. Current rises over a specified value
2. Electromagnet pulls the contacts apart
3. Breaking the circuit
91
Explain the action of earth wire
1. Earth wire provides a low resistance path to the earth
2. This causes a surge of current in the earth wire and also in the live wire
3. High current in the fuse causes it to melt and break
4. This cuts off the supply of electricity to the appliance
92
How is an e.m.f. induced in a conductor?
1. When the conductor is moved across the magnetic field, it cuts through the field lines
2. This induces an e.m.f. in the conductor
93
What does the direction of the induced e.m.f. always do?
Opposes the change that produces it
94
Describe an experiment to demonstrate electromagnetic induction
1. If a wire is passes across a magnetic field/changing magnetic field
2. A small e.m.f. is induced in the wire
3. This can be detected by a galvanometer
95
What are 3 ways to increase the magnitude of an induced e.m.f.?
1. Moving the wire faster
2. Using a stronger magnet
3. Adding more turns to the coil
96
What is alternating current (a.c.)?
One that continuously changes its direction
97
What is direct current (d.c.)?
One that is steady and only flows in one direction
98
Which electrical components produce alternating current?
Mains electricity
99
Which electrical components produce direct current?
Electric cells (batteries)
100
Describe and explain a rotating-coil generator and the use of slip rings
1. As the coil rotates, it cuts through field lines
2. This induces an e.m.f. and hence a current
3. The size of the e.m.f. could be increased by: (List)
4. Slip rings transfer current to the metal brushes whilst allowing the coil to rotate freely
101
What are slip rings in a rotating-coil generator attached to?
The ends of the coil
102
When is induced e.m.f. greatest in a rotating-coil generator?
When the coil is horizontal
103
Why is induced e.m.f. greatest when the coil is horizontal in a rotating-coil generator?
The coil cuts through magnetic field lines at the fastest rate
104
Exam tip ; )
Remember to say ‘add more turns to the coil’ and not ‘add more coils’ – if you give the second one it will be marked wrong.
105
What is a transformer?
An electrical device that can be used to increase or decrease the voltage of an a.c.
106
What can a transformer not work with?
Direct current
107
What does a step-up transformer do?
Increases the voltage of a power source
108
What does a step-down transformer do?
Decreases the voltage of a power source
109
How does a transformer work?
1. A.c. is supplied to the primary coil
2. A changing magnetic field is produced by the coil
3. The field passes through the soft iron core and secondary coil
4. Changing field in the secondary coil induces an e.m.f.
110
When is a transformer used?
High-voltage transmission of electricity
111
What is are 2 advantages of high-voltage transmission?
1. Reduces power loss
| 2. Thinner, cheaper cables can be used
112
Explain why power losses in cables are lower when the voltage is high
1. Current causes heating in wires
2. Resulting in power loss
3. Increasing voltage reduces current
4. Smaller current causes less heating in wires
5. Less power is lost
113
What is created when there is a current in a wire or solenoid?
A magnetic field around the wire
114
What do magnetic field lines created by a current in a wire or solenoid look like?
Concentric circles around the wire
115
What do strength and direction of a magnetic field created by a current depend on?
Direction and magnitude of current
116
Exam tip ; )
The strength and direction of the field depend on the size and direction of the current:
- If the current is increased, the field will get stronger
- If the direction of the current is changed, the direction of the field will change
The strength of the magnetic field is related to the distance between the field lines:
As the field lines spread out, the field gets weaker
The field lines around a wire get further apart the further they are from the wire
117
What is the direction of a magnetic field line at a point?
The direction of the force on the N pole of a magnet at that point
118
What is the effect of reversing current on a current carrying conductor in a magnetic field?
Direction of force reverses
119
What is the effect of reversing direction of field on a current carrying conductor in a magnetic field?
Direction of force reverses
120
How does a magnetic field deflect a charged particle?
1. Moving charges forms a current
| 2. The field exerts a force on the particle, deflecting it
121
How can the deflection of charged particles be demonstrated?
1. Using a cathode ray tube and a pair of magnets
| 2. Passing a collimated beam of beta particles between the poles of a horseshoe magnet
122
How can the turning effect of a current carrying coil in a magnetic field be increased?
1. Increasing the number of turns on the coil
2. Increasing the current
3. Increasing the strength of the magnetic field
123
Describe the action of a d.c. motor
1. Current passes through the coil
2. Current creates a magnetic field around the coil
3. This interacts with the field of the magnets
4. Force is exerted on the coil
5. This pushes one side of the coil up and the other down
6. Creating a turning effect
7. The split-ring commutator reverses the direction of current in the current every half turn
124
Why does the split-ring commutator reverse the direction of current in a d.c. motor every half turn?
To reverse the direction of force and keep the coil spinning
